Fighting between Cambodia and Thailand escalated along their contested border on Tuesday, as the Southeast Asian neighbours both said they would not back down in defending their sovereignty.

With each side blaming the other for starting Monday’s renewed clashes, it was unclear how or if a fragile ceasefire brokered by US President Donald Trump in July could be salvaged.

Cambodia’s influential former leader Hun Sen said his country waited 24 hours to honour the ceasefire and allow for evacuations before launching counterattacks overnight against Thai forces.

“Cambodia needs peace, but Cambodia is compelled to counterattack to defend our territory,” he said in a Facebook post, saying strong bunkers and weapons gave Cambodian forces the advantage in defending against an “invading enemy”.

In Thailand, military officials said there were clashes in five border provinces, and a Navy-led operation in its Trat Province to expel Cambodian soldiers was expected to end soon. They said Cambodia was using artillery, rocket launchers, and bomb-dropping drones to attack Thai forces.

“Thailand is determined to defend its sovereignty and territorial integrity and therefore military measures must be taken as necessary,” Defence Ministry spokesperson Rear Admiral Surasant Kongsiri told the media briefing where other military officials also spoke.

Cambodia’s Defence Ministry accused Thailand of “brutal and unlawful actions”, saying nine civilians were killed since Monday and 20 seriously injured. Thai officials said three soldiers had died in the fighting and 29 people had been injured.

Cambodian Prime Minister Hun Manet, Hun Sen’s son, said late on Monday that Thailand “must not use military force to attack civilian villages under the pretext of reclaiming its sovereignty”.

Both countries said they had evacuated hundreds of thousands of people from border areas.

Tensions have simmered since Thailand last month suspended de-escalation measures that were agreed at a summit overseen by Trump, after a Thai soldier was maimed by a landmine that Bangkok said Cambodia had recently laid.

Monday’s clashes were the fiercest since a five-day exchange of rockets and heavy artillery in July, when at least 48 people were killed and 300,000 displaced, before Trump intervened to broker a ceasefire.

In May, tensions rose following the killing of a Cambodian soldier during a skirmish, which led to a major troop buildup at the border and escalated into diplomatic breakdowns and armed clashes.

Thailand has superior military capabilities, with armed forces that dwarf its neighbour in terms of personnel, budget and weaponry, and fighter jets that have been carrying out air strikes to support its ground forces.

Thailand and Cambodia have for more than a century contested sovereignty at undemarcated points along their 817-km (508-mile) land border, with disputes over ancient temples stirring nationalist fervour and occasional armed flare-ups, including a deadly week-long artillery exchange in 2011.